Electric railway vehicles such as locomotives or rail coaches powered by an alternating-current (AC) supply line use a traction transformer and an AC/DC converter for converting the high voltage (15 kV or 25 kV) of the supply line to a direct-current (DC) link voltage of a few kV and to ensure a galvanic separation between the high voltage and the traction circuits. A DC link or bus at the DC link voltage feeds the drive or motor converters for traction or propulsion of the vehicle, as well as auxiliary converters for auxiliary energy supply. In the European Patent EP 597 409 B1, several four-quadrant actuators are each connected to a respective secondary winding of a traction transformer, the primary windings of which are being connected in parallel and connectable to a supply line and a rail. In one embodiment, two pairs of primary and secondary windings are each arranged concentrically around a first and second transformer core limb, respectively. The two limbs in turn are arranged geometrically in parallel, and their respective ends are magnetically coupled through two yokes giving rise to a transformer core of rectangular shape. A return limb is provided between the two yokes and capable of absorbing the sum of the magnetic fluxes generated by a DC current in the two secondary windings.
In modern railway vehicle concepts, the traction transformer is usually positioned outside the main casing of the vehicle, i.e. under floor or on the rooftop. In these places however, a conventional transformer with a nominal frequency of 16.7 Hz or 50 Hz causes integration problems due to its high weight and large volume. Alternative power supply systems therefore aim at replacing the aforementioned conventional transformer by additional power electronic converters based on semiconductor technology in combination with a smaller and lighter transformer operating at a higher frequency. At the expense of switching losses in the semiconductor devices, the mass and volume of the transformer as well as the total, i.e. copper and magnetic, losses in the transformer can thus be reduced, resulting in a more efficient use of the electrical power from the supply line.
In the patent application EP-A 1 226 994, a medium frequency power supply system for rail vehicles is presented, including a classical converter topology for the bidirectional conversion of a high input AC voltage to a DC output voltage. The system comprises a primary converter composed of at least three cascaded converter modules or sections electrically connected in series, one single common transformer and a single secondary converter. Each cascade module in turn is formed by a four-quadrant converter, a 3.6 kV DC intermediate stage and a resonant converter. The secondary or output converter is a resonant switched four-quadrant converter feeding the vehicle's 1.65 kV DC link. All switching elements are advanced 6.5 kV Insulated Gate Bipolar Transistors (IGBT) with an adapted gate driver technology.
Instead of passing through a DC intermediate energy storage stage, conversion from the supply line frequency to the transformer frequency can be accomplished directly by a direct AC frequency converter, also known as a cycloconverter. By way of example, DE 2614445 discloses a rectifier converting low frequency AC voltage into DC voltage via a transformer operating at medium/high frequencies that comprises, on the AC side of the transformer, an externally controlled, single-phase bridge cycloconverter. The latter comprises a single conversion stage between the low frequency of the supply line and the medium/high frequency of the transformer.
In U.S. Pat. No. 5,182,535, a summing transformer for star-delta inverter with shared flux paths on secondary legs is disclosed. EP 1113570 describes a transformer arrangement made of modular transformer units for scalable transformer output for multilevel power converters.